A principal cause of inefficiency in heat engines is the limited temperature range available in most cases, such as under 500.degree. C. in nuclear power plants. Another is the deviation of the engine cycle from the Carnot form, because any such deviation represents transfer of heat in and out of the engine at intermediate temperatures. This wastes the potential of the transferred heat to perform work for the balance of the operating temperature range. In principle, these losses are reversible, in that they would turn into thermal gain if the engine cycles were executed in reverse to pump heat, representing the absorption of ambient heat. Real engines additionally have irreversible losses caused by thermal leakage and by mechanical and thermal resistances; the last is due to the temperature drop that accompanies heat flow, similar to the voltage drop across an electrical resistor in proportion to the current. The temperature difference is evidently the "thermal motive force" driving the diffusion of heat, with Fick's law playing the role of Ohm's law for thermal resistance.
A common prescription for achieving Carnot efficiency is quasistatic operation, viz. the idea that both thermal and mechanical frictional losses should vanish when the engine is operated infinitely slowly. In the limit, however, the output power of the engine would also be reduced to zero, but the rate of quiescent thermal losses, which depend only on the available temperature difference, would generally remain unaffected. As a result, quasistatic operation is guaranteed by nature to destroy the efficiency altogether, so that the principle is as such inadequately conceived. The operating range of real engine speeds is in fact determined by the sum of the static (quiescent) and frictional (irreversible) losses, as the thermodynamic conversion must exceed this sum to sustain the operation of a real engine.
Since the irreversible losses can be reduced indefinitely in mechanical and electrical systems by improvements in design and engineering, it is the reversible losses, which have been neither small nor asymptotically reducible, that have been the main concern in thermodynamic theory. The reversible losses are determined solely by the phase space geometry of the engine cycle, and hitherto, the only general way for minimising these losses has been to select or design engines with the most efficient cycles, and to employ regeneration where possible. Opportunity is said to exist in magnetic refrigerators to approximate the Carnot cycle by shaping the medium and the magnetic poles, but the efficacy of the approach appears to be quite limited. Though dynamic control techniques have been applied to heat engines for over a century, the purpose and scope of the control has remained conservative. In automotive applications, for instance, the control over fuel injection and ignition timing is merely intended to maintain the engine efficiency, in effect preserving the engine cycle geometry, as the speed varies. The possibility of dynamically and continually modifying the cycle geometry has not been known at all in prior art, where the design principles generally call for cycles of fixed form, such as the diesel, Sterling or Otto cycles, and much of traditional thermodynamics has been designed to deal with integral segments of such cycles, like isothermal and isobaric processes. Prior art engines are incapable of emulating the phase space cycles of one another, since their cycles are fixed by construction and principle of operation.
Motivation for the present invention comes partly from the observation that the flywheel traditionally used for sustaining engine operation also constrains the piston to sinusoidal motion, but can be avoided in electrically operated heat engines, thus introducing a new degree of freedom, piston motion control, in engine theory. Engines using bulk magnetic or dielectric media do not perform well as replacements for gas engines because of the large thermal mass and the slowness of thermal diffusion in bulk solid media, as described by K H Spring in Direct Generation of Electricity, Acad. Press, 1965. The engines can be scaled to microscopic dimensions and operated at very high frequencies, however, avoiding both problems. U.S. Pat. No. 5,714,829, issued Feb. 3, 1998, entitled Electromagnetic Heat Engines and Method for Cooling a System Having Predictable Bursts of Heat Dissipation and incorporated herein in its entirety by reference, particularly describes their use in situations where the heat is generated within the medium, making large temperature changes available at high repetition rates, despite, and actually exploiting, the slowness of the diffusion of heat in solids. While the operational flexibility of these engines is noted in the above referenced Patent, the possibility and manner of almost-Carnot operation had remained undisclosed.
Importantly, these engines also bear a very close resemblance to electrical transformers, which is exploited both ways in the present invention, to apply control techniques taken from electrical and mechanical engineering disciplines in the design and operation of heat engines, and to translate the heat engine concepts of phase space and the Carnot cycle to electrical and mechanical machines. Furthermore, the special nature of heat is shown to make only a very specific difference in the dynamical analysis, which detracts very little from a purely dynamical perspective, thus providing new insight into the origin and limitations of the second law.
Accordingly, the principal object of the present invention is to provide a method for finely controlling the phase space path of real heat engines, in order to realise engine cycles of arbitrary forms in the phase space. A related object is to provide a method for obtaining near-Carnot efficiencies in real heat engines, and to make electrically operable heat engines more efficient.
Another related object is to provide a method for obtaining higher throughputs in power transformers even at low frequencies. A further object is to develop a unified, dynamical insight into and treatment of the transformation of power and heat.